Thin porous layer with open porosity and a method for production thereof

ABSTRACT

The aim of the invention is to produce a thin porous layer, with a defined porosity and also, a high strength. Said aim is achieved, whereby such a layer with open porosity is produced from a mixture, comprising a sinterable powder with a predetermined powder particle size distribution. The sintered layer is of a thickness, which corresponds to about triple the average diameter of the powder particles employed, has a pore diameter in the range from 0.01 to 50 μm and a tensile strength of in a range from about 5 to 500 N/mm 2 . The invention further relates to a method for the production of said thin porous layer with open porosity.

RELATED APPLICATIONS

[0001] This is a continuation of International Application No.PCT/EP00/09422 filed Sep. 27, 2000, which claims priority to GermanApplication No. 199 63 698.2 filed Dec. 29, 1999.

BRIEF DESCRIPTION OF THE INVENTION

[0002] The invention concerns a thin porous layer with open porosity,manufactured from a mixture containing sinterable powder, and also aprocedure for its production.

[0003] Porous bodies are required for the most varied applications intechnology. These bodies flow through a flowing medium whereby eitherreactive processes take place or the solid particles contained in theflowing medium are retained or filtered. Filter pads made from ceramicmaterial must generally be relatively thick to prevent breakage. The lowstrength and low resistance to temperature limit the use of plastics asfilter material. Metallic material is used as a porous layer in the formof tissues or fabrics manufactured from metallic fibers. Filter padsmade from pressed and sintered metallic powders are also relativelythick due to technical reasons. These filter pads must be manufacturedrelatively thick mainly because they do not exhibit the requiredstrength, particularly the tensile and shearing strength. Since thethickness does not reduce (particularly in case of extremely fine porousmaterial), the flow resistances increase.

[0004] Irrespective of the material used, the unwanted flow resistancesin case of a porous layer flowing through from a medium must be reducedby making the layer as thin as possible and the layers given adequatestrength. Thin layers, for example with thickness of about 100 μm can beproduced from metallic tissues or fabrics.

[0005] However, these are less strong, have relatively larger pores andhigh porosity tolerances. The manufacture of such type of tissues andfabrics requires correspondingly thin and also expensive threads. Hence,the tissues and fabrics manufactured from these are also expensive.

[0006] EP 0 525 325 B1 puts forth a procedure for manufacturing porous,metallic sintered workpieces, whereby a metallic powder is suspended ina carrier fluid containing a binding agent dissolved in a solvent andthe fluid adjusted such that the suspension can be poured. Thissuspension is poured into a mould. The solvent is then evaporated sothat the remaining binding agent strengthens the metallic powder in theshape given by the mould and forms a commercially viable green body.After removing from the mould, the green body is sintered in the usualway. This known procedure is preferred for manufacturing relativelythick-walled sintered parts, which, owing to their geometry, can bebetter manufactured with the pouring process than the traditionalmethod, in which metallic powder is pressed in a mould. Thin-layered,open porous parts cannot be manufactured with this procedure. The thinlayers manufactured by this procedure are brittle; they do not exhibitadequate strengths.

[0007] The invention has the task of producing a thin porous layer witha defined porosity and sufficient strength and devising a procedure forits manufacture.

[0008] The solution is a thin porous layer with open porositymanufactured from a mixture of sinterable powder with a predeterminedpowder particle size distribution. The sintered layer exhibits athickness, which corresponds to about triple the average diameter of thepowder particles used.

[0009] Moreover, the thickness mainly lies within a range of about 3times to 25 times, preferably 3 times to 10 times the diameter of thepowder particles, has a defined pore diameter in the range of about 0.01to 50 μm and a tensile strength in the range of about 5 to 500 N/mm²,preferably 20 to 400 N/mm², and more preferably 50 to 300 N/mm²,measured in accordance with DIN EN 10002.

[0010] Sinterable powders, according to the invention, refer to powdersmanufactured from metals, metallic oxides, ceramics and/or plastics.Usable metallic powders according to this invention are not only powdersmade from pure metals, but also those made from metallic alloys and/orpowder mixtures of different metals and metallic alloys. These metalsare particularly steels, preferably chromium-nickel-steels, bronzes,nickel-based alloys such as Hastalloy, Inconel or the like. Powdermixtures can also contain high melting components like platinum or thelikes. The used metallic powder and its particle size depend on thepurpose of use. Preferred powders are the alloys 316 L, 304 L, Inconel600, Inconel 625, Monel and Hastalloy B, X and C.

[0011] The abovementioned ratio of layer thickness to particle diameterensures that many layers of particles are arranged one on top of theother and holes larger than the desired pore diameter are avoided,thereby also avoiding through holes. The grain size and therefore thediameter of the used powder particles lie in a range of 0.05 μm to 150μm, preferably, in a range of about 0.5 μm to 100 μm, more preferably ina range of 0.5 μm to 6 μm.

[0012] Non-homogeneity and hollow spaces in the thin porous layer arethus positively avoided. It is also possible to influence the porosityrange up to a certain degree through the particle size of the usedsinterable powder.

[0013] The maximum thickness of the invented layer is about 500 μm; itpreferably lies in a range of about 5 to 300 μm, more preferably 5 to 18μm. Such types of thin layers with sufficient strength were notmanufactured so far. The invented layer shows remarkably low flowresistance while passing through with fluid or gaseous media and also asufficiently high strength and stiffness. Thus, the invented layer canbe used without a carrier body as films or membrane or can be boundfirmly with a carrier body. The invented layer also adjusts excellentlyto an uneven, for example bent surface on account of its retainedflexibility.

[0014] Another preferred type is the thin porous self-supporting layer.Self-supporting in the sense of this invention means that the inventedlayer can be used without any carrier body and yet does not becomefragile or brittle. Thus, it is possible to manufacture film sheets andplace them one on top of the other in many layers and if necessary, cutthem as per the requirements. Because of their self-supportingproperties, the invented layers can be used as filters and catalystmaterials wherever, for example, materials similar to paper were used sofar. The invented thin porous layers made from sinterable powders aresuperior to the familiar paper films or films made from paper-likematerial because of their distinctly higher service lives, betterbackwashing properties and a broader area of application, particularlyin view of the possible temperatures and pH-values.

[0015] The bubble-point pressure of the invented layer lies preferablyin a range of about 8×10⁶ to 2×10³ Pa, especially preferred in a rangeof about 8.6×10⁶ to 1.72×10² Pa, determined according to DIN 30911.

[0016] In another invented design, the mixture (from which the thinporous layer is manufactured) is made up of inorganic and/or organicpore forming materials. Urea is especially suitable for this purpose. Itexists in the crystalline form and therefore in defined particle sizes.Anyway, it is also possible to use ammonium carbonate and otherinorganic salts. Styropor, sucrose, gelatins and tapestry glues are theorganic pore forming materials used. However, the usual binding agentsor paraffin, which are used as auxiliary materials for reducing thefriction in the tools used in powder metallurgy, can also be used. Thesebinding agents and paraffin are not used in the usual low concentrationsbut are used in a portion of at least about 5 vol.-%, preferably morethan 12 vol.-% in the mixture for manufacturing the thin porous layer.The pore forming materials can exist in the defined particle form andparticle size and also as a solution and soluble dissolved in a solventto be used. The pore forming materials generally exist in definedparticle form and size.

[0017] The pore forming materials can be divided into two differentgroups, firstly a group of pore forming materials that serve asretainers for the mixture to be sintered for the fine pores formedlater. The other group consists of pore-forming materials that are usedas fillers usually to achieve high porosity. In the first mentionedgroup, in which the pore forming materials function as retainers, thesame are used in a particle size (grain size) that lies in the sizerange that must exhibit the fine pores contained in the thin porouslayer.

[0018] If for example, the aim is to achieve fine pores in a range of 1μm, the pore forming materials should not be considerably above or below1 μm. Thereby, it is ensured that the desired pore sizes are achieved inspite of the shrinkage process during sintering of the mixture into thethin porous layer. It must be presumed that shrinkage will take place.The use of pore forming materials as fillers is especially recommendedwhen the invented thin porous layers must have low thickness and anextremely high flow. According to the invention, it is also possible touse mixtures, particularly the above-mentioned substances, also withvarying thickness and/or sizes as pore forming materials. The inventionalso provides for combinations of pore forming materials in the mixture,in which these serve as retainers as well as fillers.

[0019] The sinterable powder contained in the mixture is made ofball-shaped and/or spattered particles. The ball-shaped particles ensurea uniform distribution of the sinterable powder and if necessary ofsubstances, particularly pore forming materials in the mixture. Thus,the sinterable powder particles do not stick together. Spatteredsinterable powders on the other hand, make feasible layers with lowerthickness and relatively larger pores for the same high strength sincethey form more and therefore better bonds with the neighboring spatteredparticles than ball-shaped particles. The sinterable powder is at leastpartially made up of short fibers. Here, metallic fibers can be takeninto account having diameters between 0.1 and 250 μm, preferably 1 μm to50 μm, and a length of lesser μm up to millimeter size, preferably in arange of 0.1 to 500 μm.

[0020] Thus, very thin sintered open-porous layers with defined porositycan be manufactured by mixing sinterable particles in the fiberstructure with sinterable particles in the ball structure in combinationwith the suspended pore forming materials depending on the purpose ofapplication. Thus the permeability of the layer is increased.

[0021] In another type, the invented layer shows a graded design. Thismeans that smaller pores exist in a separate thin porous layer on oneside than the opposite side of the porous layer. With the graded design,the flow resistance of the thin-pored layer can be adapted exactly tothe requirements. In order to retain the penetrating particles on theside of the invented layer with smaller pore diameter and to enable theflowing gas or fluid to pass easily on the opposite side of the layer inthe area of the larger pore diameter, such type of graded layers have alesser flow resistance as compared to non-graded layers. A graded designis preferred in case of a single layer. This reduces the productiontimes and costs. Moreover, separation and binding problems such asleakage are eliminated, for example with aging.

[0022] Further, the invention also concerns a procedure for themanufacture of a thin porous layer with open porosity, whereby thesinterable powder with the predetermined size distribution of powderparticles is suspended along with particles of predetermined size aspore forming material in a carrier fluid. At least one such layer isapplied on the carrier body, dried and the green layer thus formedsintered.

[0023] While sintering a green layer formed out of sinterable powder,preferably a metallic powder, the individual powder particles bindfirmly with one another whereby free spaces remain between the powderparticles. These free spaces give an open porosity with respect to thethickness of the sintered layer such that the layer becomes permeablefor flowing media.

[0024] After the sintering process, the layer can either be removed fromthe carrier or further processed along with it. Filter cartridges can bemanufactured in a simple way in a single procedural step, i.e.manufacturing the sintered layer without carrier body without priorseparation and then applying it on a filter cartridge. This is possiblebecause the firm bonding achieved between the carrier body and the layerafter the sintering step as long as the carrier body is able to bindwith the sinterable powder, whereby the bond can be further improved byabout 3 to 8 times by using highly sinter-active metallic components.

[0025] There exists a dependency between the particle size and thetarget pore size of the finished sintered layer. The mechanical strengthof a porous sintered layer also depends on the particle size. Finer thepowder particle, higher is the mechanical strength. Since the resistanceto flow also depends on the thickness of the finished sintered layerdepending on the medium (fluid or gaseous), porous layers with largerpore sizes have a lower mechanical strength than a porous layer withsame thickness with smaller pore size. Thus, the mechanical strength oflayers with larger pore size can be increased merely by increasing thelayer thickness and thereby the resistance to flow.

[0026] This problem can be solved only with the help of the inventedprocedure by suspending the sinterable powder along with particles withpredetermined size or size distribution as pore forming material in thecarrier fluid.

[0027] This carrier fluid is then applied in at least one layer on thecarrier body, dried and sintered. In another procedural step, thesintered layer can be removed from the carrier body or the carrier bodybound firmly with the sintered layer, for example by the sinteringprocess itself. By adding the pore forming material, it is possible toachieve a defined pore size. The sinterable powder particles distributedin the suspension and thereby in the applied thin layer and the poreforming material combine to form a lattice structure. Thus, a definitepore structure can be defined on the basis of the size or sizedistribution of the pore forming material, practically independent ofthe size and size distribution of the sinterable powder. This also meansthat the size and size distribution of the sinterable powder can bechosen exclusively keeping in view the mechanical strength, i.e. veryfine sinter powder can be used. On the other hand, the pore formingmaterial can be chosen keeping in the view the required porosity.

[0028] Since the sinterable powder, preferably metallic powder and thepore forming materials are suspended in a carrier fluid, particles ofmaterials with thickness lower than that of the sinterable powder can beuniformly distributed and suspended for the pore forming material,irrespective of the varying thickness of materials and in keeping withthe consistency of the suspension. Thus, it is possible to form a layeron a carrier body, in which the particles that exist as pore formingmaterials are uniformly distributed.

[0029] If the pore forming materials serve as retainers, these mustevaporate under the effect of heat, i.e. during the sintering processpreferably without any residue and remain inert as against the materialof the sinterable powder even at sintering temperatures. As a result, nochemical reactions take place between the pore forming material and thesinterable material, which is, as a rule, a metal.

[0030] Evaporable solvents like ethanol, methanol, Toulon,trichlorethylene, diethyl ether and also lower molecular aldehydes andketones can be used as carrier fluid especially at temperatures below100° C. Paraffin, shellac as well as polymer compounds can be used asbinding agents, whereby preferably polyalkylene oxide or polyglycols,especially polyethylene glycols can be used. Polyalkylene oxide andpolyglycols are used preferably as polymers and/or copolymers withaverage molecular weights in a range of 100 to 500,000 g/moles,preferably 1,000 to 350,000 g/moles, and more preferably 5,000 to 6,500g/moles.

[0031] In another design of the invented procedure, the portion of thepore forming materials in the suspension more or less matches with thedefined pore volume of the porous layer to be produced. Thus it ispossible, for example, to specify a defined porosity of the porous layerto be produced for a very fine and thereby highly sinter-activesinterable powder by specifying the volumetric details when the size ofthe particles of the pore forming material is given.

[0032] The consistency of the suspension adjusted on the basis of thecarrier fluid mainly depends on the application of the suspension on thecarrier body. The suspension can be adjusted to a thick-fluidconsistency while pouring, if necessary with subsequent coating of theexcess of the poured suspension layer. A thin fluid consistency must beprovided for the so-called film pouring or spraying. The carrier fluidcan be formed with a binding agent liquefied with an evaporable solvent.Hereby, it is ensured that the green layer has sufficient strengthresulting from the bonding of individual powder particles one below theother with the binding agent.

[0033] In another form of the invention, a suspension is used forachieving a graded layer design. This suspension comprises of poreforming materials of different densities and/or size suspended insolvent. Here, there arises a balance within the layer while adding thesuspension to the carrier body. As a result, the lighter particles ofthe pore forming materials collect in the upper area of the layer,whereas the heavier particles of the pore forming material collectcloser to the side facing the carrier body of the layer. Obviously, thisbalance is influenced by the grain size of the used sinterable powder.If particles from a material with different sizes are used in thesuspension as pore forming material then, for example, the finishedsintered layer shows a gradient with respect to the pore diameter of thesame. This is particularly advantageous since the flow resistance canthus be further reduced.

[0034] In a specific form of the invention, the suspension is applied inmany thin partial layers one after the other on the carrier body.Hereby, the individual partial layers can be made up of an identicalsuspension. It is possible to use suspensions with different sizedistributions for the individual partial layers for the powder usedand/or different particle geometries and/or different powders. Thisallows for example, the use of powders that give an especially goodporosity to the fully sintered layer on one hand and to manufacture atleast one layer that shows especially favorable, for e.g. catalyticproperties in its composition for the purpose of application.

[0035] It is necessary to dry the coated partial layer before coatingthe next partial layer. Hereby, it is ensured that the first coatedpartial layer is properly fixed so that it is not deformed by thecoating procedure, e.g. spraying of the next partial layer.

[0036] On the other hand, the remaining portion of the solvent in thepreviously coated, dried partial layer ensures that even the nextpartial layer is properly bound with same packing density and thefinished green layer has the desired strength.

[0037] In another form of the invention, the partial layer is sinteredbefore applying the next partial layer. This procedure is especiallyadvantageous when powder made from different sintered materials, forexample having deviating sintering temperatures, is used for amultiple-layer design. Thus, it is possible to first apply the partiallayer containing the powder with the maximum sintering temperature onthe carrier body, and after sintering of the first layer in thecorresponding sequence to apply the subsequent partial layers with lowersintering temperatures and subject them to the sintering process. Thishas the advantage that the desired porosity of the individual partiallayers remains intact because of the individual sintering steps. Thisporosity would have been lost if the suspension was coated with such aheterogeneous powder mixture in one layer and sintered in just one step.In this process, the remaining low sintered powder parts would havedensely sintered because of the high sintering temperatures required foronly one portion in the powder mixture. As a result, the porosity wouldbe further lost.

[0038] If the carrier body is also part of the finished part and if,correspondingly the porous layer is fixed firmly with it, another formprovides for the suspension to be applied on at least one of the wallsof the carrier body made from sinterable material, dried and the greenlayer subsequently sintered firmly on to the carrier body.

[0039] The carrier body can be a sintered molded part or even a poroussintered molded part with a coarse pore structure. The suspension can beapplied once again through thin layer required, spraying or immersion onthe upper surface of the carrier body. The layer can be coated on theouter wall and/or on the inner wall depending on the purpose ofapplication.

[0040] If the carrier body is formed by a pipe-shaped carrier body, theinvented procedure provides for the rotation of the carrier body aroundthe axis of the pipe during the application of the suspension and atleast during part of the drying period. This ensures that the thicknessof the layer remains intact till the fixing of the suspension as a greenlayer on the carrier body. Therefore, it is necessary that thesuspension outlet be moved in a defined way against the surface inaddition to rotation, particularly during thin layer pouring andspraying.

[0041] Porous layers applied as films or membrane or on a porous carrierbody are particularly suitable as filter material and also as microfilters, provided that they have the required porosity. In case ofimpermeable carrier bodies, such a component can be used as a catalystor membrane reactor, provided that it has the required powdercomposition and porosity, for example mixed with palladium or coated. Itis further possible to use the layer as friction material, e.g. on ironbase. It can be applied on a friction surface of a synchronous body forgears.

[0042] The invented porous layer can also be used in filter pipes andfilter cartridges having a length of 10 mm to 1,500 mm. It is alsopossible to manufacture filter cartridges that exhibit a porous coatingon the front side. Further, filter cartridges can be manufactured with asintered flange that does not have any welded joints.

[0043] With the help of the invented procedure, it is possible toimprove the permeability of filters while reducing the filter-activelayer depending on the porosity. By reducing the thickness of thefilter-active layer, the pressure loss can be distinctly reduced forconstant permeability.

[0044] According to this invention, thin porous layers enable flow ratesfor gaseous medium such as air ranging from 1 to 1500 m³/hm² at adifferential pressure of e.g. 100 mbar. For fluids such as water, theflow rates at a differential pressure of, e.g. 100 mbar are 0.1-to30-m³/hm². The permeability coefficient is about 0.002×10⁻¹² to 3×10⁻¹²m² for a layer thickness of 50 to 500 μm, measured according to DIN ISO4022.

[0045] A thin porous layer was manufactured having a thickness of 15 μm.The carrier fluid was manufactured from isopropyl alcohol, in which 1weight %—with respect to the quantity of powder used—of polyethyleneglycol having an average molecular weight of 6,000 g/moles wasdissolved. Inconel metallic powder was used as powder, which had anaverage diameter of about 1 μm. Urea was used as pore forming material,which had an average diameter of about 2 μm. The above components weremixed for 3 hours in a mixer and finally sprayed on a plastic film. Theratio of powder to pore forming material was about 1:1, similar to thatof powder to carrier fluid. The mixture was dried at room temperaturefor 24 hours and then sucked from the plastic film and sintered at atemperature of up to 950° C. in a sintering oven for 10 hours.

[0046] The thin open-porous film so obtained had a tensile strength of284 N/mm². The pore structure was uniform, whereby the pores had anaverage diameter of about 2 μm and the porosity was about 50%.

What is claimed is:
 1. Thin porous layer with open porosity,manufactured from a mixture containing sinterable powder with apredetermined powder particle size distribution, whereby the sinteredlayer has a thickness which is at least triple the average diameter ofthe used powder particles, a defined pore diameter in a range of about0.01 to 50 μm and a tensile strength in a range of about 5 to 500 N/mm².2. Thin porous layer in accordance with claim 1, marked by a maximumthickness of about 500 μm.
 3. Thin porous layer in accordance with oneof the above claims marked by self-supporting properties.
 4. Thin porouslayer in accordance with one of the above claims marked by abubble-point pressure in a range of about 8×10⁶ to 2×10³ Pa.
 5. Thinporous layer in accordance with one of the above claims marked by thecontent of inorganic and/or organic pore forming material in themixture.
 6. Thin porous layer in accordance with one of the above claimsmarked by a graded design.
 7. Procedure for manufacturing thin porouslayer with open porosity in accordance with one of the claims 1 to 6,whereby the layer is made up of a mixture containing sinterable powderand the sinterable powder with a predetermined size distribution ofpowder particles is suspended along with particles of the defined sizeas pore forming material is suspended in a carrier fluid. It is appliedin at least one layer on a carrier body, dried and the green layer thusformed is sintered.
 8. Procedure in accordance with claim 7 marked bythe correspondence of the portion of pore forming materials in thesuspension to the metallic layer to be produced in about the definedpore volume.
 9. Procedure in accordance with claim 7 or 8 marked by theforming of the carrier fluid by the binding agent liquefied with asolvent.
 10. Procedure in accordance with one of the claims 7 to 9marked by the suspension of pore forming materials of differentdensities and/or sizes in the solvent for obtaining a graded layerdesign.
 11. Procedure in accordance with one of the claims 7 to 10marked by the application of the suspension in many partial layers oneafter another on the carrier body.
 12. Procedure in accordance with oneof the claims 7 to 11 marked by drying of the earlier partial layerbefore the application of the next partial layer.
 13. Procedure inaccordance with one of the claims 7 to 12 marked by the sintering of theearlier partial layer before application of the next partial layer. 14.Procedure in accordance with one of the claims 7 to 13 marked by theapplication of the suspension on the carrier body by the process of thinlayer pouring, spraying or immersing.
 15. Procedure according to one ofthe claims 7 to 14 marked by the application of the suspension on atleast on one of the walls of a porous, preferably pipe-shaped carrierbody made from sinterable material, dried and the green layer thusformed subsequently firmly sintered on the carrier body.
 16. Procedurein accordance with one of the claims 7 to 15 marked by the rotation ofthe pipe-shaped carrier body around the axis of the pipe duringapplication of suspension and at least during some part of the dryingperiod.
 17. Using a thin porous layer with open porosity in accordancewith one of the claims 1 to 6 as filter material, catalyst, membranereactor, friction substance, filter cartridge and/or filter pipe.